The present invention relates to a voltage-controlled sweep oscillator system and more particularly to an oscillator system having two phase-locked loops which sweep through a plurality of stabilized discrete step frequencies.
Those concerned with the development of sweep oscillators have long recognized the need for a stepped frequency generator with exceptional frequency stability. The present invention fulfills this need.
An example of a prior art sweep oscillator utilizes a plurality of crystal oscillators of increasingly higher frequencies which are divided into odd and even groups of four oscillators each. Dual selecting circuits alternately select in numerical sequence one oscillator output from each of the groups. A delay circuit delays the step command one full period before triggering the even group binary counter. The outputs of eight sampling gates are connected in common to provide a serial output.
However, a separate crystal oscillator is necessary for each individual frequency which this device is to generate. Accordingly, for any application requiring generation of a large number of frequencies, this device could be modified accordingly only by utilization of a large number of crystal oscillators, which introduces practical difficulties and expense. Similarly, variation of the frequencies generated could only be accomplished by replacing the appropriate crystal oscillators.
Another type of prior art oscillator system, a swept oscillator system, utilizes a phase-locked VCO loop which provides a stepped frequency output. However, this oscillator cannot produce each frequency produced as an unchanging frequency, inasmuch as a sweep oscillator system must change from frequency to frequency, and the VCO loop cannot accomplish this all in one step. Thus, unwanted transitional frequencies, as well as the desired individual frequencies, would appear at the output of this oscillator.